Congestive heart failure (CHF) is a condition in which a weakened heart cannot pump enough blood to body organs. Heart failure may affect either the right side, left side, or both sides of the heart. As pumping action is reduced, blood may back up into other areas of the body, including the liver, gastrointestinal tract, and extremities (right-sided heart failure), or the lungs (left-sided heart failure). Structural or functional causes of heart failure include high blood pressure (hypertension), valvular heart disease, congenital heart diseases, cardiomyopathy, heart tumor, and other heart diseases. Precipitating factors include infections with high fever or complicated infections, use of negative inotropic drugs (such as beta-blockers and calcium channel blocker), anemia, irregular heartbeats (arrhythmias), hyperthyroidism, and kidney disease. The weakened heart is also susceptible to potentially lethal ventricular tachyarrhythmias. One factor that contributes to heart failure is asynchronous activation of the ventricles wherein mechanical contraction is not properly coordinated thus compromising cardiac function.
One particularly promising technique for addressing heart failure is ventricular resynchronization therapy, which is a technique that seeks to normalize the asynchronous activation of the ventricles by delivering synchronized pacing stimulus to the ventricles using pacemakers or ICDs equipped with biventricular pacing capability. More specifically, the relative timing of pacing pulses delivered to the left and right ventricles is synchronized so as to help to improve overall cardiac function. This may have the additional beneficial effect of reducing the susceptibility to life-threatening tachyarrhythmias. Typically, an interventricular delay value is adjusted by a physician to “resynchronize” the ventricles and improve cardiac function for the patient. The interventricular delay specifies the time delay between delivery of a right ventricular (RV) pacing pulse and a left ventricular (LV) pacing pulse. The RV-LV interventricular delay may be positive, i.e. the RV pulse is delivered before the LV pulse or may be negative, i.e., the LV pulse is delivered before the RV pulse. Ventricular resynchronization therapy is one form of cardiac resynchronization therapy (CRT). Ventricular resynchronization therapy, CRT and related therapies are discussed in, for example, U.S. Pat. No. 6,643,546 to Mathis et al., entitled “Multi-Electrode Apparatus and Method for Treatment of Congestive Heart Failure”; U.S. Pat. No. 6,628,988 to Kramer et al., entitled “Apparatus and Method for Reversal of Myocardial Remodeling with Electrical Stimulation”; and U.S. Pat. No. 6,512,952 to Stahmann et al., entitled “Method and Apparatus for Maintaining Synchronized Pacing.”
Unfortunately, when fitting a patient with an implantable biventricular pacing device, it can be difficult to pass a left-side lead into the coronary sinus vein, or the smaller final destination veins, or to keep it in stable position. Accordingly, there is a need for alternative techniques of applying electrical stimulus to the left ventricle, particularly for use in delivering CRT but also for use in delivering other forms of biventricular pacing therapy as well. U.S. patent application Ser. No. 10/408,198 to Kroll, entitled “Implantable Cardiac System with Master Pacing Unit and Slave Pacing Unit,” filed Apr. 3, 2003, is directed to solving these and other problems. This patent application is incorporated by reference herein. In the system described by Kroll, a satellite or “slave” pacing device is mounted epicardially on the left ventricle for use in conjunction with a primary or “master” pacing device having a RV pacing lead. The primary pacing device delivers all RV pacing pulses, whereas the epicardial satellite pacing device delivers all LV pacing pulses. In this manner, a lead need not be implanted within the left ventricle, thus simplifying implantation and reducing any risks associated with LV lead placement.
In one specific example described by Kroll, the satellite pacing device detects RV pacing pulses delivered by the primary device, then delivers LV pacing pulses following a predetermined time delay. Using this system, CRT therapy may be delivered so long as the RV-LV interventricular delay is positive, i.e. so long as the right ventricle is to be paced prior to the left ventricle. In order to allow the left ventricle to be paced before the right ventricle, the patent application of Kroll sets forth an alternative implementation wherein the primary pacing device transmits control signals directly to the satellite pacing device for controlling the satellite device to deliver a LV pacing pulse prior to each RV pulse delivered by the primary pacing device. In other words, once the primary device has determined when it will deliver the next RV pacing pulse, it sends a control signal to the satellite device controlling that device to deliver an LV pacing pulse at a point in time shortly prior to the upcoming RV pacing pulse. In one of the specific examples described therein, the control signals are transmitted via wireless telemetry.
Although the systems and techniques described by Kroll represent a significant improvement over predecessor systems, there is room for further improvement. In particular, it would be desirable to provide a primary/satellite pacing system that does not require control signals to be transmitted via telemetry from the primary device to the secondary device to allow LV pulses to be delivered prior to RV pulses. To provide for transmission of control signals, the primary device and the secondary device must be equipped with suitable signal transmission/reception telemetry components. This increases the cost, size, weight and complexity of both of the devices and causes a greater drain on their power supplies. Also, when implemented using wireless telemetry, wireless signals generated by external devices can potentially interfere with signals transmitted from the primary device to the satellite device, possibly resulting in a failure of the satellite device to properly deliver needed LV pacing signals. In addition, the wireless signals of the implanted system can potentially interfere with the operation of external devices.
Moreover, many patients who might benefit from CRT already have RV pacing devices implanted therein. It would be desirable to instead provide a “stand-alone” satellite pacing device that could simply be used in combination with the existing device, so as to eliminate the need to remove the existing device and replace it with one specifically equipped with the capability for transmitting the aforementioned control signals to the satellite pacing device. A significant problem however is that, as already noted, it is often necessary for LV pacing pulses to be delivered prior to RV pacing pulses. Without the capability of the primary device to communicate the aforementioned control signals to the satellite device, it does not appear that the satellite device would be capable of operating in situations wherein LV pacing pulses need to be delivered prior to RV pacing pulses.
The above-referenced parent patent application to Poore et al. described an invention that solved these problems. Briefly, techniques set forth in Poore et al. provide for delivering cardiac pacing therapy to the heart of a patient using an epicardial left ventricular satellite pacing device in conjunction with an otherwise conventional pacemaker having a right ventricular pacing lead. Right ventricular pulses generated by the pacemaker are detected by the satellite pacer and analyzed to determine the timing pattern employed by the pacemaker. Then the timing pattern is used to specify the delivery times of epicardial left ventricular pulses so as to be synchronized with right ventricular pulses.
By first determining the timing pattern of the primary pulses, supplemental pulses may thereby be delivered in synchronization with expected upcoming primary pacing pulses. Synchronization may be subject to a predetermined time delay, which may set, for example, to specify that the supplemental pulses are to be delivered shortly prior to corresponding primary pulses. In this manner, synchronization is achieved between supplemental pacing pulses and primary pacing pulses while incorporating a “negative” primary-to-supplemental time delay, but without requiring transmission of control signals directly from the primary pacing device to the satellite device. Alternatively, as needed, the delay may be set to specify that the supplemental pulses are to be delivered shortly after corresponding primary pulses, or simultaneous therewith. In any case, by having the satellite pacer decode the timing pattern of the primary pacer, biventricular pacing therapy may be conveniently delivered using an otherwise conventional non-biventricular pacemaker in combination with the improved epicardial satellite pacer.
Although the techniques of the parent application are effective, additional or alternative techniques for solving the problems described above would also be desirable. In particular, it would be desirable to provide an alternative technique that permits the primary pacer to communicate primary pulse timing information to the satellite pacer (1) without requiring that the satellite pacer to decode the timing pattern of a sequence of primary pacing pulses and also (2) without requiring the primary pacer to transmit control signals via wireless telemetry to the satellite pacer. It is to this end that the present invention is primarily directed.